This invention relates to a seal for hydrogen-cooled generators and more specifically to a ventilated trickle seal provided as a low leakage seal for hydrogen-cooled generators that relies on a relatively wide region of brush seals to reduce hydrogen leakage. The invention further relates to a low leakage seal in which any leakage gas is disbursed using shop air to ventilate the region of gas release.
Hydrogen has been widely used as a coolant in a variety of rotary electric machines due to its desirable thermophysical properties including low density and specific heat. However, a disadvantage of using hydrogen is that the hydrogen purity must be maintained above its explosive limit (74% hydrogen in air). Therefore, a significant consideration for safe operation of hydrogen cooled rotary machines, such as turbogenerators is designing highly reliable and efficient hydrogen seal systems therefor. In a hydrogen cooled turbogenerator, hydrogen seals are utilized both to seal high pressure hydrogen at the interface of the rotating shaft and to prevent air from entering the casing and from developing an explosive mixture with the hydrogen. Such sealing must be provided at all static joints of the machine as well as at the interface between the rotor and the stator at both ends of the machine.
Another consideration is the total amount of hydrogen that must be supplied to the machine. While this is not as serious a concern as an explosive situation, it represents a constant expense that adds to the total cost of generating electricity.
Current hydrogen-cooled generators utilize an oil film seal to create a zero leakage seal across the interface between the stator and the rotor at both ends of the machine. See in this regard U.S. Pat. No. 6,082,740; copending application Ser. No. 09/586,044; co-pending application Ser. No. 09/586,045, and co-pending application Ser. No. 09/668452, filed Sep. 25, 2000; the disclosures of each of which are incorporated herein by this reference. While the seal has essentially no leakage across it, oil that enters the machine casing becomes saturated with hydrogen gas, resulting in hydrogen consumption.
Several manufacturers, such as Siemans and ABB, use a complex system involving seal rings isolated from the casing by a secondary set of seals. This two-layer seal system reduces the amount of oil that enters the machine casing, but it requires more space within the machine as well as pumps and plumbing resulting in a high initial cost. Other manufacturers, such as General Electric, use a much simpler system with bearing lube oil being used to feed the seal oil system and a relatively simple set of rings where the hydrogen side oil can directly mix with casing gases. This system thus has a lower initial cost. However, due to the hydrogen consumption noted above, ongoing hydrogen costs would be higher than, for example, the more complex systems noted above. A need remains, therefore, for a seal for hydrogen cooled generators that can maintain non-explosive hydrogen conditions at all times, minimize hydrogen consumption, and reduce or eliminate complex auxiliary support hardware.
The invention is embodied in a seal provided around a shaft extending through a pressurized cavity. In contrast to current seal strategies in which zero leakage is allowed outside of the system, the seal of the invention is adapted to reduce flow to/from the cavity to a level where the risk of creating an explosive mixture of gases is minimized. Thus, with the seal of the invention a minimum amount of leakage is permitted through the seals
The seal of the invention provides a plurality of brush seal structures to minimize the flow of hydrogen out of the cavity. In an embodiment of the invention, a series of brush seals are provided to interface with a rotary shaft so as to define a seal for minimizing the flow of hydrogen out of the cavity. In one preferred embodiment, the seal is composed of first and second brush seal assemblies, each comprised of a plurality of brush seal structures, one assembly being provided on each side of a step in the exterior surface of the rotary shaft. In another embodiment, spring(s) are provided to urge the brush seal structures into sealing engagement with the rotor.